Two stage drying of nonmetallic ore precedent to electrostatic separation



April 14, 1959 2,881,916

C. C. COOK TWO STAGE DRYING OF NONMETALLIC ORE PRECEDENT Filed Sept. 7, 1954 TO ELECTROSTATIC SEPARATION 2 Sheets-Sheet 1 I l/ II If I I II I 1/ I/ f/ l/ l/ I/ 1/ 1/ INVENTOR.

ATTORNEY April 14, 1959 c. c, coo 2,881,916

TWO STAGE DRYING OF NONMETALLIC ORE PRECEDENT TO ELECTROSTATIC SEPARATION Filed Sept. 7, 1954 2 sheet -sheet 2 %B,PL m TAILS 6 B PL IN CONCENTRATES "IN V EN TOR.

ATTORNEY.

United States tent TWO STAGE DRYING OF NONMETALLIC ORE g igilgEDENT TO ELECTROSTATIC SEPARA- Charles C. Cook, Lakeland, Fla., assignor to International Minerals & Chemical Corporation, a corporation of New York Application September 7, 1954, Serial No. 454,524 12 Claims. (Cl. 2093) This invention relates to the preparation of dry, substantially slime free granular ores. More particularly, it I relates to the preparation of ore as feed material for electrostatic and electrodynamic separators. Still more particularly, it relates to a method of drying phosphate ore which has been rendered substantially slime free by washmg.

It is well known that conductors can be separated from nonconductors by electrodynamic methods. Further, certain materials exhibiting differential electrical elfects even though electrically rated as nonconductors or insulators, may be separated by electrostatic methods.

In all of these separations, there are two factors of primary importance, namely, the feed material must be dry and the feed material must be in a condition to accept electrical charges when drying is completed.

The degree of dryness will vary with the characteristics of materials. When dealing with granular material, the

moisture content must be reduced below 1% and generally below 0.2% moisture content.

As the moisture content is increased, the efiectiveness of electrostatic separation is markedly reduced because particles exhibit false conductivity or because particles, if they acquire differential electrification, quickly lose the electrical charge through leakage of the charges to contacted equipment.

Even when the ore is dry, efiective separations may be precluded by the presence of fine material termed secondary slimes which are produced by the processing treatment of the ores initially washed slime free.

In the preparation of materials in small batches for separation by the above mentioned methods, no particular problem arises since the material may be held in an electric oven without agitation indefinitely, until dry. When dealing with continuous, large scale operations wherein for example, 400 to 500 tons of feed must be dried per hour, the cost of electric oven drying would be prohibitive. continuous basis, resort was had to otherequipment such as rotary drum dryers and to so-called fluodryers with unsatisfactory results, due either to nonuniformity of drying, or formation of slimes, or both.

-It is therefore a primary object of this invention to overcome the shortcomings and disadvantages of the above described methods of separating feed materials for electrostatic or electrodynamic separators.

It is another objective of this invention to produce a dry ore feed which is not contaminated With deleterious quantities of secondary slimes.

It is another object of this invention to provide for electrostatic separations an adequately dry ore feed in commercially feasible quantities.

It is still another object of this invention to provide a method wherein the heating of ore does not give rise to any apparent effects which lower electrostatic separation effectiveness.

. It is still a further object of this invention to provide a'method for drying nonmetallic soft ores such as phos- In an attempt to dry granular ore material on a Patented r. 14, 1959 phate rock under conditions to avoid formation of second aryslimes.

These and other objects of the invention will be appar ent to those skilled in the art from the description hereinafter presented.

It has been discovered that the lack of response to the forces in an electrostatic field of relatively soft and attritionable nonmetallic deslimed comminuted ore materials which have been dried under agitating conditions may be cured, if the ore is reduced to final dryness in the absence of any appreciable agitation. So long as the moisture content of the ore is relatively high, violent agitation of the drying ore apparently has little or no effect detrimental to the electrostatic separation characteristics of the ore. On the other hand, once the moisture content has been reduced to or approaches a minimum moisture content predeterminable for each type of ore, agitation during continuing dryingoperations produces a detrimental etlect upon the electrostatic separation which becomes progressively greater as the moisture content is reduced. Since, when drying by heating methods, high heat transfer coefficients are not attained unless the solids are agitated, the preferred mode of drying is to heat the ore to dryness in two stages under conditions whereby in the first or preliminary drying stage the ore is agitated while conditions are maintained to insure that the moisture content is not reduced below the allowable minimum for agitated drying. The partially dry ore is then reduced to final dryness in the absence of any appreciable agita tion. Absence of any appreciable agitation is not intended to mean that movement of the particles can not be tol erated. Particle movement in the secondary drying stage, however, must be of such a character that appreciable rubbing contact or attrition of the particles is avoided. I

The extent of removal of moisture content in the preliminary drying stage before eifects deleterious to the separation become apparent is more critical the higher the degree of concentration sought. When drying phosphate rock material, for example, if the increase in bone phosphate of lime content sought is not too great, drying to a moisture content in the preliminary stage of about 0.35% to about 0.5% can be tolerated. If however, the bone phosphate of lime content is to be raised from, say feed content of 20% to 60% or higher, the moisture content cannot be reduced appreciably below about 0.5%- 0.6% in the preliminary drying stage.

In carrying out the drying, the solids may be subjected to indirect heating or to direct contact with hot gases or gaseous combustion products. In the preferred mode of operation comminuted ore is directly heated by suspending the solids in a combination of heated air and gaseous combustion products. In this preliminary heating stage solids are agitated and are transferred from a gas contact zone to a solids dropout zone typical of which is a solids separation cyclone. In this preliminary heating stage the moisture content of the ore is reduced from about 12% to a moisture content in the range between about 0.35% and about 1.5%. This partially dried ore is then dried to the low moisture content necessary for good electrostatic separations, generally to less than about 0.1%. This secondary drying is under conditions minimizing agitation or substantially nonagitating in nature. In this latter stage of drying, indirect heating methods are preferred such as passing the solids through heat exchange units of the type hereinafter described, although equipment for direct contact of gases and solids such as tunnel or pan dryers can be utilized.

In order to electrostatically separate the components of an ore, it is necessary to grind to economical liberation. Economical liberation in the case of a potash ore such as sylvinite requires grinding to less than about 8 mesh size. Liberation may be accomplished for Florida phosphate rock by grinding to less than about 24 mesh size, or by sizing out the 24 mesh material in the matrix, which is the common practice.

Particles of the comminuted phosphate ore generally fall in the range between 24 mesh and'200 mesh, with the feed material for the electrostatic separation preferably having a particle size in the range of -24 mesh 'plus 100 mesh.

Control over the moisture content of this comminuted ore material is obtainable in a .preliminarydrying stage, as in a suspended solids dryer -by control of inlet gas temperature, gas velocity and solids retention time. By intercontrol of mass velocity and inlet gas temperature, a relatively constant gas outlet temperature may be maintained. Temperatures'generally in the range of about 2000 F. to about 2400 F. are utilizedinasmuch as they are easily handled in commercially available equipment. Outlet temperatures for a suspended solids dryer generally are maintained in the range of about 180 F. to about 235 F., and preferably in the range between about 200 F. and about 220 F. When utilizing outlet temperatures in the general range above, material having a moisture content'between about 1.5% and about 0.5% can be-secured for partially dry phosphate rock.

In order to elfectively separate ore by electrostatic methods, it is desirable that the moisture content be less than about 0.2% and preferably that the moisture content be less than about 0.1%. While ore having a moisture content between about 0.3% and about 0.5% can be separated, the degree of beneficiation which can be accomplished is considerably lower than optimum, when dried under agitating conditions. Moisture content in excess of 0.5% adversely affects beneficiation and rapidly reduces the beneficiation to uneconomical proportions. Regardless of the method of drying, e.g., under agitating or nonagitating conditions, two stage or single stage drying, when the moisture content of the material at the time of separation exceeds 1.5%, the effectiveness of electrostatic separation as a method of beneficiation is destroyed and in the case of phosphate ore, for example, the phosphate content of the gangue product rapidly increases.

" On the other hand, after the moisture content is V brought into the range where effective separations can be made, it rapidly approaches a minimum of 0.35% by weight beyond which the drying cannot be continued while agitating the solids without again reducing the effectiveness of separation as demonstrated relative to phosphate ore by the rapid decline in bone phosphate of lime content of theconcentrate and rapid rise in the bone phosphate of lime content of the gangue product.

After the comminuted material has been reduced in the preliminary drying step to a moisture content in the range of about 0.5% to about 1.5%, the partially dry comminuted ore must be subjected to a final drying which brings the material to the point of dryness where optimum separation can be obtained in the electrostatic separation step. The secondary drying, therefore, is carried out in equipment where the dryingis maintained under substantially nonagitating conditions as, for example, in a tunnel dryer, pan dryer ovens or equivalent apparatus. Naturally, the drying temperature will vary with the type of drying equipment utilized.

' After the comminuted material is reduced to the desired low moisture content, the particles are induced to accept an electrical charge. As distinguished from other methods in common use where, for example, material is not merely polarized as in the case of pyroelectric crysrats, the charging of the particles may be and preferably is carried out in the absence of an electrical field. In the performance ofthis step the particles are diiferentially electrified so that one component of the ore carries an electrical charge of different character or of different magnitude from the other components of the comminuted mixture. Difierential electrification may be created by 4 utilizing the contact potential phenomenon such as by frictional or rubbing contact between particles either in contact with a donor plate or not. When the quantities of different ore components of a substantially uniform particle size are not widely disproportionate, contact potential charging may be effectively carried out by agitation or movement of the mixture. Under such conditions a'donor plate is not critical to the operation. When charging concentrates, particularly of relatively high purity, contact potential may give only weak charging of one component of the mixture and at this stage use of a donor element is generally advantageous. By grounded donor plate is meant an element of low work function which readily exchanges electrons with the ore particles when the plate is grounded to the earth, and for optimum charging would have a work function between the two components which it is desired to separate. One method of accomplishing this result is to convey ore particles such as phosphate ore having a temperature in the range of approximately 160 F. and approximately 350 F. to a feeder of lead, zinc, aluminum, copper, tin, iron, and the like, grounded to the earth by an electrical conductor. The ore particles are caused to flow over the chute surface in agitated action such as is caused by a vibrator.

Dielectric or nonconducting materials vary in their acceptance of an electrical charge. In general it may be stated that electrons flow from the material of lower work function to the material of higher work function, with the result that, for example, silica particles become negatively charged. The charge on the silica particles is at least definitely more negative than the charge on most of the remainder of the ore particles, the majority of which particles of the remainder exhibit a positive charge.

Particles which have acquired a charge may then be separated as, for example, by being fed as free falling bodies between the electrodes of one or more electrostatic separating units; i.e., in a path normally not in contact with said electrodes.

The strength of the electrostatic field maintained between electrodes which will effectively alter the path of falling particles varies with the particle size of the ore fed to the separator. The voltage may vary from 5,000 volts per inch of distance between electrodes in separating material of relatively fine particle size in the range of approximately mesh to approximately 200 mesh, to 15,000 volts per inch of distance separating electrodes handling coarse particles. In all such discussions of field strength, it must be borne in mind that corona discharges which ionize air are to be avoided. In general, it is preferred to operate with a total impressed difference in potential of about 70,000 to about 90,000 volts, although voltages as low as about 20,000 and as high as 200,000 are utilized on occasion. This voltage should be maintained at a high direct voltage potential substantially free of alternating current components; i.e., filtered DC. current should be low in the so-called A.C, ripple. A standard supply of DC. voltage may also be obtained Without expensive filtering apparatus by the use of such equipment as radio frequency power supply.

The invention will be more fully understood from the following description in conjunction with the drawings in which:

Figure 1 is a horizontal cross sectional view of a vertical heat exchanger type secondary drying unit.

Figure 2 is a vertical cross sectional view of the same secondary drying unit.

Figure 3 is a fragmentary vertical cross sectional view of the tube bundle unit showing the steam vents.

Figure 4 is a plot showing the change in effectiveness of separation with changes in the mode of drying.

With reference to Figures 1 to 3, inclusive, the numeral 10 indicates a shell preferably cylindrical in shape. Within shell 10 are spaced tubular members 11 generally of cylindrical shape. Tubes 11 are secured to the top andbottom into headers 12 and 13 which close the top and bottom of shell 10. Within tubes 11 are positioned, by means of spiders, vent pipes 14 of relatively small diameter compared to tubes 11. Vent pipes 14 are provided at vertically spaced intervals with apertures 15 which are preferably directed downwardly and radially transverse to the vertical axis of shell 10. Protective cones 16 may be provided to prevent material from entering the apertures 15.

3 Shell is provided adjacent header 12 with a conduit member 17 which is an outlet for heating gases passing through the jacket formed about tubes 11 by shell 10. Shell. 10 is also provided adjacent the header 13 with a conduit member 18 communicating with the interior of shell 10 which is an inlet for heating gases delivered from furnace 19. Header 12 is provided with a cone member 20 positioned with its vertical axis in alignment with the longitudinal axis of the shell 10. Cone 20 directs feed particles into the tubes 11. Below the header 13 is positioned a solids removal apparatus such as a rotary table 21 whose ribs or rakes unload a predetermined quantity of solids from beneath the tube outlets. Table 21 delivers heated solids to the plant feed belt 22.

In the operation of this heating unit it is generally preferred to feed partially dry ore particles over cone 20 at a rate designed to maintain the solids level in tubes 11 a few inches below the top. Eight inch diameter tubes can 'be operated to heat from 2 to tons of solids per hour, per tube. The solids accumulate in a bed almost filling the tubes and moving by gravity, continuously or intermittently depending upon the character of the solids removal apparatus positioned at the solids outlet of the heating unit. Heating units of this general type can be modified to feed solids through the shell outside the tubes instead of through the tubes, if desired. Such heating units also need not be vertical, but can be positioned with'their central axes transverse to the vertical. With a 4 inch spacing between tube outlets and table feeder and at a table rate of 5 r.p.m., each 8 inch tube will heat 1.0 ton per hour of phosphate solids from 200 F. to 380 F. in approximately 9 minutes.

The invention will be further illustrated by the following examples given by way of illustration and without any intention to limit the invention thereto.

Example I Florida phosphate pebble obtained as washer debris having by'screen analysis, particles in the size range of -:24 plus 100 mesh was subjected to scrubbing with water in order to complete the desliming operation. The scrubbing was performed in an agitating unit at 70% solids and dewatered in a Hardinge drag classifier.

I The solids or comminuted ore from the classifier had a water content of about This wet ore was stored untilthe ore had drained to a moisture content of about 12%. The drained solids were divided into four portions. Three portions were fed to a fiuodryer by a pressure controlled screw feeder. The solids were suspended in a 12 inch diameter stack through which passed 60icubic feet of gases per minute, measured at about 70 F., of'air heated to 2,500 F. plus the products of combustion.

' "Three portions of the feed were separately treated as hereinafter described. Curves were plotted from the data obtained and graphically presented in Figure 4.

The first portion (curve A) fed to the fiuodryer was withdrawn from the fluidizing stack at a temperature of approximately 320 F., and had a moisture content of approximately 0.2%. This partially dry ore was then dried to a moisture content of less than 0.1% in an electric oven maintained at 300 F. The second portion (curve'B) of said solids was fed to the fiuodryer and suspended under identical conditions as portion one, except that the solids were withdrawn from the fluidizing stack whenat a temperature of approximately 215 F.

and having a moisture content of approximately0.45 This partially dry ore was then dried to a moisture content of less than 0.1% in the electric oven, maintained at 300 F.

The third portion was fed to the fiuodryer and suspended under identical conditions as portion one, except that the solids were withdrawn from the fluidizing stack when at a temperature of approximately F. and having a moisture content of approximately 1.3%. The 1.3% moisture content material was split into two parts; one part (curve C) was dried to a moisture content of less than 0.1% in the electric oven, maintained at 300 F., the other part (curve D) was electrostatically separated without further drying.

A fourth portion (curve B) of ore was dried to less than 0.1% moisture in the electric oven maintained at 300 F. without any preliminary drying.

Each of the products was controlled to have a temperature of approximately 200 F., and then was delivered to a Syntron vibrating trough which had the metal trough thereof grounded to the earth by an electrical conductor. The Syntron vibrating trough discharged the particles as free falling bodies between electrodes spaced approximately 10 inches apart and maintained at a potential gradient of approximately 8,000 volts per inch. The rate of feed of the dry comminuted material was approx imately one ton per hour, per linear foot of electrode.

The plot of bone phosphate of lime in the tail versus the bone phosphate of lime content of concentrates shows the separation effected for products varying in moisture content and degree of agitation during drying.

Curve A shows that material dried to too low a moisture content while being agitated gives a very limited response to an electrostatic field. Curve B shows that drying ore to approximately 0.45% moisture under agitating conditions and then to less than 0.1% moisture content under nonagitating conditions gives a product of excellent separation characteristics. Curve C shows the improved separation which can be etfected when the material of 1.3% moisture content has been further dried under nonagitating conditions to less than 0.1% moisture content. This curve shows that separations approximating oven drying; i.e., drying under nonagitating conditions may be obtained by two stage drying, provided agitated conditions are not maintained when approaching the final low moisture content. Curve D shows that phosphate matrix ore of approximately 1.3% moisture has too high a moisture content to be eifectively beneficiated by electrostatic methods. Curve B shows the separation eifected with ore dried under conditions of negligible agitation. Further, Curve B shows that the bone phosphate of lime content of the tails does not increase over a wide range of bone phosphate of lime content of the concentrate.

Example II Drained phosphate solids from the same stock pile as the material for Example I was obtained were fed to a conveying type dryer. In this dryer solids were heated by hot air plus combustion products having a temperature of about 1500" F. In this dryer the suspended solids were transferred vertically about 20 feet in a 10 inch diameter stack through which was passing 2500 cubic feet of gas per minute. Gas and solids were separated in a cyclone separator, the outlet gas temperature being controlled to be approximately 205 F. giving a moisture content in the cyclone recovered solids of approximately 0.8%.

This partially dry ore was fed to a solids heating unit of the type described hereinabove. Hot gasses at a temperature of about 2000 F. entered the jacket of the shell and issued from the jacket at a temperature of about 1500 F. Solids collected by the cyclone entered the tubes of this solids heating unit at a temperature of approximately 200 F. at a rate of approximately 1.0 ton means-1a 7 per hour per tube. The 12 :tons of solids collected from the twelve tubes at the time of discharge onto the table feeder averaged approximately 380 .F. and had a moisture content below 0.1%.

This 380 F. phosphate feed entered the first or rougher electrostatic separation stage at a temperature of approximately 325 F. The solids passed over a Syntron vibrating trough which had the cast iron trough thereof grounded to the earth by an electrical conductor. The Syntron vibrating trough discharged the particles as freely falling bodies between electrodes spaced inches apart at the top and diverging so as to be 30 inches apart at the bottom. These electrodes were maintained at a potential gradient of approximately 10,000 volts per inch of distance between electrodes. The rate of feed of the .dry comminuted material to the electrostatic separation unit was approximately one ton per hour per linear foot of electrode.

Results of the rougher separation were as follows:

Material Percent Percent B.P.L. Weight Concentrate from the rougher being at a temperature of 200 F. was fed to a second separation unit identical to that used for the rougher separation. The voltage gradient also was maintained the same. Products in this second or concentrate cleaner separation were as follows:

Material Percent Percent B.P.L. Weight Material Percent Percent Weight B.P.L.

Feed 56.8 Concentrat 71. 6 16.8. Tail '46. 5 24.2 (closed out; with rougher feed).

The concentrates from the rougher and cleaner sections when combined formed a product having a B.P.L. content of approximately 71.8 B.P.L., and an insoluble content of approximately 5.2%. Recovery of phosphate was about 78% on the basis of bone phosphate of lime content.

This application is a continuation'in-part of co-pending application Serial No. 277,226, filed March 18, 1952, entitled Drying Methods, now abandoned.

Having thus fully described and illustrated the character of the invention, what is desired to be secured and claimed by Letters Patent is:

1. The method of preparing relatively soft and attritionable nonmetallic ore for electrostatic separation which comprises heating the deslimed comminuted ore under agitation conditions to a moisture content of not less than about 0.35% by weight and heating thejpartially dry material to final dryness under substantially nonagitating conditions.

2. The method of preparing relatively soft and attritionable nonmetallic ore for electrostatic separation which comprises heating the deslimed comminuted ore under agitation conditions to a moisture content approaching about 0.35% by Weight minimum and completing removal of moisture to less than 0.1% by weight under substantially nonagitating conditions.

3. The method of preparing relatively soft and attritionable nonmetallic ore for electrostatic separation which comprises heating the deslimed comminuted ore in two separate stages, the first stage being under agitating conditions and lowering the moisture content to about 0.35 by weight minimum and heating the partially dry material to less than 0.1% moisture in a second stage where substantially nonagitating conditions are maintained.

4. The method of preparing phosphate pebble ore for electrostatic separation which comprises heating the deslimed comminuted ore under agitating conditions to a moisture content in the range between about 0.35% and about 1.5% by weight and heating the partially dry ma terial to a moisture content less than 0.1% by weight under substantially nonagitating conditions.

5. The method of preparing phosphate pebble ore for electrostatic separation which comprises fluodrying the deslimed comminuted ore to a moisture content in the range between about 0.35% and about 1.5% by weight and heating the partially dry material to final dryness under substantially nonagitating conditions.

6. The method of preparing phosphate pebble ore for electrostatic separation which comprises fluodrying the deslimed comminuted ore to a moisture content in the range between about 0.35 and about 0.75% by weight and completing the removal of moisture to a content of less than 0.1% by weight in a multiple hearth roasting furnace.

7. The method of preparing phosphate pebble ore for electrostatic separation which comprises heating the deslimed comminuted ore under agitating conditions to a moisture content in the range between about 0.35% and about 1.5 by weight with gases having a temperature in the range between about 2000 F. and about 3000 F. at the inlet to the agitating zone, and completing the removal of moisture to a content less than 1.1% under substantially nonagitating conditions.

8. The method of beneficiating nonmetallic ore comprising oomminuting the ore, washing the comminuted ore free of secondary slimes, drying the washed ore in two stages, the first stage effecting moisture removal under agitating conditions to a moisture content approaching 0.35 by weight minimum and completing the moisture removal under nonagitating conditions in the second stage, inducing the dry particles to accept an electric charge, subjecting the charged ore as free falling bodies to the attracting and repulsing forces of a high potential electrostatic field, and recovering a product rich in the desired ore component.

9. The method of beneficiating phosphate pebble ore comprising comminuting the ore, washing the comminuted ore free of secondary slimes, drying the washed ore in two stages, the first stage effecting moisture removal under agitating conditions to a moisture content approaching 0.35 by weight minimum and completing the moisture removal under nonagitating conditions in the second stage, inducing the dry particles to accept an electric charge, subjecting the charged ore as free falling bodies to the attracting and repulsing forces of a high potential electroe static field, and recovering a product of high phosphate content.

10. The method of beneficiating phosphate pebble ore comprising comminuting the ore, washing the comminuted ore free of secondary slimes, heating the washed me in two stages, the moisture removalin the .first stage being efiected by the passage of gases having a temperature of approximately 2,500 P. at the inlet to the agitating zone until reduction of the moisture content of the material to within the range between about 0.35% and about 0.75% by weight, completing the moisture removal under non-agitating conditions in the second stage maintained at approximately 300 F., passing the dry particles while at a temperature in the range between about 200 F. and about 300 F. over a donor element to induce the particles to accept an electric charge, subjecting the charged ore as free falling bodies to the attracting and repulsing forces of a high potential electrostatic field having a potential gradient in the range between about 5000 volts per inch and about 15,000 volts per inch of distance separating the electrodes and recovering a phosphate rich product.

11. The method of preparing relatively soft and attritionable nonmetallic ore for electrostatic separation which comprises conveying deslimed comminuted ore suspended in hot gases to a solids gas separation zone under conditions to lower the moisture content of the solids to a point within the range of about 0.35% and about 1.2%, and passing the partially dry solids while under substantially nonagitating conditions as a bed moving by gravity in a restricted zone in heat exchange relationship to hot gases outside restricted zone having temperatures capable of reducing the moisture content of the solids to less than about 0.1%.

12. In a method of preparing relatively soft and attritionable nonmetallic ore for electrostatic separation the step comprising heating a deslimed comminuted ore under agitating conditions to partially dry the ore and lower the moisture content thereof to less than about 0.35% by weight and then heating the said partially dried ore in a slow moving bed under substantially nonagitating conditions to reduce the moisture content to less than 0.1% by weight.

References Cited in the file of this patent UNITED STATES PATENTS 940,190 Provost Nov. 16, 1909 1,088,357 Morse Feb. 24, 1914 2,090,418 Johnson Aug. 17, 1937 2,197,865 Johnson Apr. 23, 1940 2,350,209 Clark May 30, 1944 2,442,513 Sackett June 1, 1948 2,593,431 Fraas Apr. 22, 1952 2,738,875 Le Baron Mar. 20, 1956 2,754,965 Lawver July 17, 1956 2,782,923 Cook et al Feb. 26, 1957 

1. THE METHOD OF PREPARING RELATIVELY SOFT AND ATTRITIONABLE NONMETALLIC ORE FOR ELECTROSTATIC SEPARATION WHICH COMPRISES HEATING THE DESIRED COMMINUTED ORE UNDER AGITATION CONDITIONS TO A MOISTURE COMMINUTED OF NOT LESS THAN ABOUT 0.35% BY WEIGHT AND HEATING THE PARTIALLY DRY MATERIAL TO FINAL DRYNESS UNDER SUBSTANTIALLY NONAGITATING CONDITIONS. 